FAUNAL AND PALEOENVIRONMENTAL ANALYSIS OF THE MIDDLE SANTONIAN CODY SHALE IN THE SOUTHERN BIGHORN BASIN, HOT SPRINGS COUNTY, WYOMING, USA

During the Coniacian to Santonian, the Western Interior Seaway (WIS) of North America expanded to its greatest extent since the Cenomanian/Turonian Sea Level Rise, leading to siliciclastic and carbonate deposition across its western and eastern parts, respectively. Most faunas from this interval have been described from lower diversity distal offshore facies deposited across the Great Plains, while correlative assemblages from more proximal siliciclastic facies of the WIS are less well understood. Here, we present paleoenvironmental and faunal analysis of a new invertebrate assemblage from the middle Santonian Clioscaphites vermiformis Biozone of the Cody Shale in the southern Bighorn Basin, Wyoming. The fine-grained clay-rich lithology of this site suggests deposition in a proximal offshore environment. This depositional interpretation is supported by paleogeographic reconstructions, which indicate that the locality was deposited over 200 km east of the western shoreline of the WIS. The benthic fauna is dominated by abundant epibenthic inoceramids, ostreids, and serpulids, rare infaunal bivalves, as well as the demersal ammonite Clioscaphites. These faunal components, along with abundant vertically migrating cephalopods (Baculites, Glyptoxoceras, Eutrephoceras) and rare, actively mobile nektic ammonites (Texanites), belemnites (Actinocamax), Osteichthyes, and Chondrichthyes, suggest a well-oxygenated water column and sediment-water interface. This assemblage is significantly more diverse than those found in correlative, more distal offshore localities in the Niobrara Formation across the Great Plains, supporting an onshore-offshore environmental and community gradient within the WIS.